Re: [Qemu-devel] [PATCH] net: prevent sending packets while guest is stopped

[Jason Wang <jasowang@redhat.com>]

On 09/02/2014 10:23 PM, Stefan Hajnoczi wrote:
> Do not modify guest memory or devices when the guest is stopped.
> Currently the netdevs still send packets while the guest is stopped if
> their file descriptor was being monitored for write (e.g. the socket
> write buffer filled before the guest was stopped).
>
> Netdevs call qemu_flush_queued_packets() when the file descriptor
> becomes writable again.  Don't resume packet processing when this
> happens.
>
> Instead we flush queues when the guest resumes.
>
> Cc: qemu-stable@nongnu.org
> Reported-by: Michael S. Tsirkin <mst@redhat.com>
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> ---
> Note this fixes the transmit side.  The receive side was recently fixed in
> "net: Forbid dealing with packets when VM is not running".
>
>  net/net.c | 30 ++++++++++++++++++++++++++++++
>  1 file changed, 30 insertions(+)
>
> diff --git a/net/net.c b/net/net.c
> index 6d930ea..74ec07a 100644
> --- a/net/net.c
> +++ b/net/net.c
> @@ -47,6 +47,7 @@
>  # define CONFIG_NET_BRIDGE
>  #endif
>  
> +static VMChangeStateEntry *net_change_state_entry;
>  static QTAILQ_HEAD(, NetClientState) net_clients;
>  
>  const char *host_net_devices[] = {
> @@ -506,6 +507,11 @@ void qemu_purge_queued_packets(NetClientState *nc)
>  
>  void qemu_flush_queued_packets(NetClientState *nc)
>  {
> +    /* Guest memory and devices must not be modified while stopped */
> +    if (!runstate_is_running()) {
> +        return;
> +    }
> +

Consider migration case, this will prevent
sent_cb(virtio_net_tx_complete) from being called at source. Since we
don't migrate queue and async_tx. This may lead a interrupt lost in
destination after migration?

(Looks like virtio_net is the only user that uses async sending, not
sure why this is needed)